Advancing mechanization of agriculture and the ever-increasing sizes of arable fields which are brought under cultivation in single units, together with larger and more powerful harvesters wherein several coupled-together implements must be pulled together across fields and up slopes, demands ever greater tractive effort from the tractor or other pulling vehicle. A modern tractor must be capable of exerting more than twice the previously required tractive capability than was previously required when cultivating the soil where deeper soil loosening is increasingly required and where it is simultaneously desired to use implements having a greater working width. As a result, larger and longer tractors have been built. It was intended to meet the above demands by having increasingly wider and higher (larger diameter) tires and, in part, with the help of engageable and disengageable four-wheel drives.
However, it has been found in practice that fields do not respond like hard-surfaces, stable roads where heavy loads can be pulled without difficulty by using powerful, heavy vehicles. The wet soil of a field is excessively compressed by high ground pressures per unit area and, as a result of the high "slip" necessarily caused by the great tractive effort of the drive wheels, the already compressed soil is kneaded which experience has shown to be highly damaging. It is known that such kneading causes serious damage to the soil structure.
In particular, the water retention capacity of the soil is destroyed as is the activity of soil organisms and bacteria. This leads to reduced yields and sickly plants which are plagged by pests and fungal diseases and which must then be treated with chemical substances which are prejudicial to the environment. Such treatment leads to environmental pollution including over-fertilization such that the runoff from fields adds excessive phosphates, nitrates and the like to fresh water bodies, resulting in damage to both human and animal users of these water supplies.
Heavy tractors have been equipped with caterpillar-type tracks for reducing the compression of the soil and to simultaneously increase the pulling power. However, such tractors are only suitable for very heavy work and cannot economically be used for light work.
Swiss Pat. No. 190,576 proposes making the vehicle wheels removable and replacing them with rocker arms with endless track bands or belts. In the case of a light tractor, having a total weight of 1700 kg or so, this does, in fact, reduce the ground pressure per unit area, but the pulling power is only slightly increased because its weight is too slow.